The development of forming tools for producing a metal part is complex, expensive and time-consuming. Such forming tools may be used in numerous metal forming operations, such as stamping, deep drawing, stretching, tube hydroforming, sheet hydroforming, impact extrusion, warmforming, rod and tube extrusion, heading, fineblanking, forging, tube rod and bar drawing, wire drawing, spinning, roil forming, stretch forming, tube and pipe bending, blanking and piercing, coining, swaging, press bending, ironing and flanging.
Forming tool designers typically use a computer-aided design system, usually incorporating finite-element analysis (FEA) models, to generate initial tool geometry for producing a particular metal part. Such FEA models may be used to control a computer-aided manufacturing (CAM) system to manufacture the actual forming tool. Once formed, this tool typically requires modification by a tool and die maker to perform as intended. FEA models include assessment of friction forces on the to and its addenda as it is being used to form the required shape.
In the design process, the tool designer must select and input to the FEA model a friction parameter relating to the friction force anticipated to he experienced in the tool during the forming process. Such friction parameter is a function of the coefficient of friction (COF) between the tool, the part blank (i.e. the unformed metal piece) and the selected lubricant. Generally speaking, the lubricant will be selected to be compatible with the friction specifications anticipated for the forming process and also with downstream processes such as welding and painting. Furthermore, a manufacturing facility typically has several lubricants readily available and the lubricant selected for the FEA modelling may be specified based on the intended manufacturer's preference.
Typically, a tool designer relies upon friction parameter values which have been determined. empirically from friction force data collected using a certain category of tribotests in which the lubricant is replenished during the test to maintain a fixed film thickness. These types of tribotests are herein referred to as “non-lubricant depleting” (NLD) tribotests The NLD tribotest most commonly relied upon by tool designers for sheet metal forming tools is the Drawbead Simulator (DBS). This test as used for input data for a process involves a series of steps where a lubricated metal strip is drawn through dies causing a series of bending and straightening steps that return the strip to the original orientation. The test uses the particular tool material, blank material and lubricant to determine a coefficient of friction to be used in the FEA process.
Because of the inherent properties of the lubricant in combination with a particular blank metal, tool material (including any pertinent tool coatings or surface treatment) and the geometry of the NLD tribotest, the actual thickness of the lubricant film varies from case to case and is unknown. The friction data produced by such an NLD tribotest thus represents a COF for one particular (but unknown) film thickness. In reality, however, metal forming is a dynamic process where the lubricant film thickness (and thus friction force) changes through a wide range as the lubricant is subjected to the action of compressive forces and sliding that greatly vary in different regions of the formed part.
The generalized performance of a typical lubricant as it moves through a full range of lubricant film thickness and different stages of lubricant depletion varies considerably. With reference to FIGS. 1 and 2 (showing the various stages of lubricant depletion of a typical lubricant), for relatively thick layers of lubricant, the COF is near its minimum and adjacent surfaces essentially slide past each other on a cushion of lubricant; this is the “hydrodynamic” stage of lubricant depletion. As the lubricant layer thins, perhaps allowing for increasing metal-to-metal contact, the COF starts to increase; this is a “mixed-film” stage of lubricant depletion (“MF stage”). In the case of many combinations of lubricant and tool and blank materials, the COF increases in a generally linear manner through the MF stage. As the lubricant layer continues to thin past the MF stage, the lubricant starts to break down through the “boundary”, “EP activation” and final “breakdown” stages of lubricant depletion. Problems can occur in the EP activation and Breakdown stages beyond the MF stage including material buildup, cold welding and excessive wear. The “boundary” stage is sometimes included as part of the MF stage as the characteristics of the stages are closely related.
In metal forming processes, successful lubrication is critical to ensure acceptable finished part specifications, including surface quality, and to reduce maintenance of the forming tools. The geometry of an NLD test apparatus, including the DBS, is designed for the purpose of determining a COF, based on the specific lubricant and blank and tool materials, from which a friction parameter input to the FEA model can be derived. It is generally understood and assumed that the COF measured in a DBS test will be in the MF stage of the lubricant. As a result, the DBS in particular has for well over 20 years been the tribotest of choice with tool designers as the preferred means of determining friction data and COF with different process parameters in metal forming, such as blank and tool materials, tool coatings and surface treatment and lubricant.
Current FEA models do not take into account the dynamic nature of the changing lubricant film thickness, COF and resulting friction force. Instead, in current PEA models, it is necessary to select a single friction parameter value to represent the overall process. At present, the tool designer will select a friction parameter determined by an NLD tribotest (usually a DBS tribotest).
Using the accepted current assumptions, it is common that initial prototype parts produced by a forming tool will not meet the design specifications. This in turn requires trial-and-error modifications of the forming tool (potentially requiring many hours of welding and grinding) followed by more prototype production and, if necessary, further tool modification. This iterative trial-and-error tool manufacturing process involves a tool and die maker and often many months to successfully yield a tool capable of producing parts which meet the design specifications. The cost and delay can be substantial and the required time to complete the process is difficult to predict.
As a result of the above, there remains a need for an FEA-model-controlled tool design and manufacturing method and system that improves the initial tool geometry.